System for digital fabrication of graded, hierarchical material structures

ABSTRACT

A system to fabricate hierarchical graded materials includes a reservoir to contain a material to be deposited, a print head connected to the reservoir to allow the print head to receive the material to be deposited, the print head having a mixing section, and an actuator connected to the print head, the actuator configured to actuate the print head in six axes of motion.

TECHNICAL FIELD

This disclosure relates to fabrication of multi-scale heterogeneousmaterials, more particularly to fabrication of these materials fromcolloidal elements using a microfluidic print head and a roboticdeposition system.

BACKGROUND

Fabricating hierarchical, multi-material structures offers tremendouspromise for controlling and tailoring material properties. An examplethat highlights both the potential and the challenges of hierarchical,multi-material structures is discussed in “Simultaneously High Stiffnessand Damping in Nanoengineered Microtruss Composites,” ACS Nano, Mar. 12,2014, pp. 3468-3475. The microtruss composite possesses thecontradictory design goals of high stiffness and high damping. Thesestructures have sound absorbing qualities in cars and airplanes whilealso being strong. Fabrication of these structures involves the growingof carbon nanotubes (CNTs) from a steel plate and coating with aluminausing traditional semiconductor processes, followed by multipledeposition steps to build a polyurethane/clay layer on the CNTs andfinally encapsulation with polyurethane. This material is graded withthe interface between the polymer and CNT microtruss composed of anano-composite polymer that has intermediate properties, which isimportant for overall mechanical robustness. However, this method is notscalable to large areas or digital.

US Patent Publication 2011/0079936, to Oxman, explores concepts ondigitally printing graded materials using dye in a FDM (fused depositionmodeling) print head. The disclosure teaches ‘on the fly’ blending ofmaterials for creating continuous gradients in material properties.However, this concept does not have the capability to build multiscalestructures or heterogeneous material structures required to achieve thefull potential of architected materials.

SUMMARY

Embodiments include a system to fabricate hierarchical graded materialsinclude a reservoir to contain a material to be deposited, a print headconnected to the reservoir to allow the print head to receive thematerial to be deposited, the print head having a flow focusing section,and a mixing section, and an actuator connected to the print head, theactuator configured to actuate the print head in six axes of motion.Further embodiments have a print head including a reactive bondingsection and a fluidic particle alignment section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph illustrating an example of hierarchical gradedmaterial.

FIG. 2 shows an embodiment of a system to fabricate hierarchical gradedmaterials.

FIG. 3 shows a block diagram of a print head in a system to fabricatedhierarchical graded materials.

FIG. 4 shows an embodiment of a flow focusing section of a print head.

FIG. 5 shows an embodiment of a reactive bonding section of a printhead.

FIG. 6 shows a diagram of a four roll mill.

FIG. 7 shows a diagram of an embodiment of an inlet/outlet elementequivalent to a four roll mill.

FIG. 8 show flow diagrams of modulation of a principal axis oforientation.

FIG. 9 shows views of an embodiment of an inlet/outlet component.

FIGS. 10-11 views of an exit nozzle of a print head.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion will use several terms that have particularmeanings when used here. A ‘hierarchical material’ is a material havingstructures spanning multiple orders of magnitude of length scale. A‘graded material’ is a material having a continuous and smooth spatialgradient of composition. A ‘digital material’ has controlled spatialcomposition and form. The unit of a digital material is a voxel, whichis a three-dimensional droplet or characteristic volume of material. A‘multi-material composite’ is a material composed of a suspendingmaterial, usually a polymer, also known as a resin, and a dispersedparticulate reinforcing material. The reinforcing material may compriseof colloidal clays, carbon nanotubes, or macroscopic additives likecarbon fibers or ceramic platelets.

Merely as an example to illustrate materials that have some of theseproperties, polyurethane-based composites have been formulated that arehierarchically reinforced with hard polymeric crystalline domains,laponite nanoplatelets and alumina microplatelets at progressivelylarger length scales. By controlling the concentration of such buildingblocks within the polymer matrix, it is possible to change the elasticmodulus of polyurethane-based composites within a very broad rangespanning over four orders of magnitude. FIG. 1 shows a graph of thismaterial. Extreme gradients in mechanical properties within the samematerial approaches the range of elastic moduli found in highly gradednatural composites and is far beyond the span of properties obtainablein metal/ceramic functionally graded materials. This graph is from apaper, “Biological and Bioinspired Composites with Spatially TunableHeterogeneous Architectures,” Advanced Functional Materials, 2013, byProfessor A. R. Studart, (www.wileyonlinelibrary.com).

FIG. 2 shows an embodiment of a system to digitally fabricatemulti-scale, or hierarchical, heterogeneous materials using novelmicrofluidic print heads coupled to colloidal manipulation chemistry anda robotic deposition system. The system may take many forms and theembodiment of FIG. 2 is merely one example. The embodiment of FIG. 2 hasa reservoir 12 in which functional colloidal particles are provided tothe print head to be printed on the final substrate. An umbilical orconduit such as 14 allows the material to flow from the reservoir to alocal reservoir attached to the print head. A controller 16 willregulate the flow of the material to the print head and the motion ofthe actuator 18.

The actuator 18 has the capability to move the print head 20 in 6 axesof motion, forward/backward, up/down, left/right, on the threeperpendicular axes, as well as the three rotational axes, also known aspitch, yaw and roll. This facilitates both the mixing and orienteddeposition as will be discussed in more detail later. The print head 20will typically have multiple nozzles, such as 22, to allow deposition ofthe materials.

The mixer that has a desired output of a continuous profile ofcomposition versus time. The changes in composition occur at a spatialresolution equal to a voxel. This change in composition has implicationsfor the volume residence time and residence time distribution. The‘residence time’ is the time between the functional materials enteringthe print head and exiting the nozzle. The residence time varies fordifferent materials as they move through the print head at differentrates. The ‘residence time distribution’ (RTD) of a chemical reactor isa probability distribution function that describes the amount of time afluid element could spend inside the reactor. The below discussion ismerely an illustrative example and is in no way intended to limit theembodiments disclosed here.

In this example, an output of the printer is assumed to be a line ofmaterial with a square cross section that has a height and width equalto the voxel size, which is 100 microns (1e-4m). The discussion alsoassumes that the linear deposition rate is 1 cm/s or 1e4 microns/sec, or100 voxels/second. Each voxel has a specified composition state,specifically by a volume percentage of material A and B, as examples.Each voxel must have a composition that is homogeneously mixed anddefined. “Homogeneously mixed” means materials A and B are homogeneouslydistributed through the voxel such that there are no localized areas inthe voxel with a higher or lower amount of A and B than what is desiredin the voxel overall. “Defined” means that the specified composition andthe actual composition of the voxel are the same.

For the voxel to be homogeneously mixed, the mixer should be able toprocess a single voxel of a material that is a 100×100×100 micron cubeand fully mix it in the time required to deposit the material at a rateof 100 voxels/sec, 0.01 seconds. Alternatively, the mixer must be ableto ‘queue’ a number of voxels and mix them in such a way that each100×100×100 micron cube of material that exits the mixer is at thespecified composition state desired in the part buildup. This latterconfiguration relaxes the mixing speed requirement, the time for mixingnow is the residence time of a specific voxel in the mixer, which is thenumber of voxels in the mixer divided by the deposition rate. If themixer processes 100 voxels at a time, the mixing time becomes 1 second,which is more reasonable, especially for viscous liquids.

Mixing multiple voxels presents challenges for typical mixing techniquessuch as static or screw mixers. While this mixers achieve highlyeffective mixing through multiple folding, stretching and cuttingoperations, they also have a wide range of fluid residence times. Thisrange of residence times results from the tortuous streamlines andnear-zero flow regions within the mixer. Particles or fluid elementsthat sample these parts of the flow field typically have long residencetimes. In practice, this means that a square pulse of fluid dyetypically exits the mixer with a residence time distribution that has along tail of material that samples these low flow or tortuous pathregions.

This long tail is problematic as it pollutes the voxel immediately afterthe one being mixed. Over many voxels, the residual material from eachvoxel mixing operation using a mixer with a typical RTD will cause thevoxel composition to deviate significantly from what is specified, andthe voxel will not be defined. Therefore, the mixer should mix materialsin such a way that the RTD has no tail. Quantitatively, the RTD must beon the order of the voxel residence time itself. In this example, theRTD should be 0.01 seconds.

There are several mixing techniques that could attain both of thesecriteria. For example, high parallelized mixing chambers that areactuated using flexible valves, such as those employed by Quake, oracoustics could achieve both small mixing volumes with mixing and tightresidence time distributions as defined above. However, these techniqueshave not been demonstrated using highly viscous liquids of interest toadditive manufacturing.

The reservoir 12 contains functionalized particles. Using anisotropicplatelets or other colloidal building blocks, different functionalgroups on different faces of the particle can drive interactions andassembly in a prescribed manner. The anisotropic particles may consistof platelets, chiplets, wires, etc., all of which will be referred tohere as fibers. Control of the selection of the functional group on thefibers enables explicit cross-linking of colloids to each other incontrolled morphologies. Using one reactive group and reaction of theresulting fibers with the resin of another achieves strong interfacialcontact between the particles and matrix to provide mechanicalperformance.

As an example, the system can direct assembly of engineeredmulti-particles structures where different particles are connected in aprescribed manner as dictated by keying chemistry. This would allowcreation of multi-scale reinforcement materials using chiral particles,with the chirality a function of both the particle anisotropic andkeying chemistry. Chirality may enable large-scale ordering of theparticles in a manner analogous to collagen fibrils seen in bone.

Examples of printable particle-reinforced composites can be found in apaper by Brent Compton and Jennifer Lewis “Printing of LightweightCellular Composites,” Advanced Material, Vol. 26, pp. 5930-5935, 2014and in the Army Research Lab technical report “Micro-compositefabrication via Field-Aided Laminar Composite (FALCom) Processing”,ARL-TR-6106, September 2012. In the FalCOM system, fibers are depositedusing a nozzle and then aligned using electric fields. This process isnot ideal since the electrical fields required are high (kV/mm) and theparticle alignment times in viscous polymeric media are on the order of1-10s which slows overall processing speed. Lewis's work uses a muchsimpler extensional flow nozzle to attain particle alignment in adirection parallel to the nozzle. In the embodiments here, arbitraryparticle alignment is obtained fluidically, combining the bestattributes of both of these prior art systems for controlledreinforcement deposition.

FIG. 3 shows a more detailed view of the print head 20. The print headcomprises a mixer section 30, a possible flow focusing section 32, andoptional reactive bonding and fluidic particle alignment sections, 34and 36. Each of these will be discussed in more detail in turn.

FIG. 4 shows a more detailed embodiment of an embodiment of a flowfocusing section 32. The flow focusing section here provides merely oneembodiment of a flow focusing section. In the embodiment of FIG. 4, theflow focusing section creates a coaxial flow of a material stream ofsheathing fluids entering the section through channels 40 and 42, whichwill sheath the main fluid flow coming into channel 44. The sheathingoccurs at region 46 where the flows come together. This highlyextensional flow aligns particles, which will be important as discussedin more detail further.

The flow focusing section may be followed by a reactive bonding section32 as shown in FIG. 5. This section may be optional depending upon thematerials used. One result of the reactive bonding section is a linkingof the particles to form particles of varying lengths. The bonding willtypically result from application of a stimulus 52. The stimulus mayinvolve the application of heat or light, such as with a pulsed laser orlocalized heating. By modulating the duration of the stimulus, and theflow of the building block material, the fibers may vary in length fromseveral hundred nanometers to millimeters. This achieves parity withexisting Stable Free Radical Polymerization (SFRP) approaches. Thisapproach may be applied to various types of colloids for whichfunctionalization chemistries are known including silica, clays,graphene and hydroxyapatite.

Prior to the flow focusing section and the reactive bonding section, amixer 30 mixes up the base materials. Modulating resin composition orfiber content may be achieved by using multi-layer mixers, which alsomake particle alignment possible. For fibers that are smaller than theorifice size (approximately 50-100 microns), fluidic systems for fiberorientation can control the straining axes of the flow. One of the mostprevalent methods to control the straining axes in a flow is to use afour roll mill as shown in FIG. 5. In a four roll system such as 54 thestraining axes of the flow are controlled by changing the rotation rateof the four cylinders. A linear flow ranging from pure rotation to pureextension can be generated. Fibers align along the principal strainingaxis in a purely extensional flow. By adding more rotation to the flow,the extensional axes are brought closer together.

As mentioned above, the flow focusing, reactive bonding, and fluidicparticle alignment sections are optional. The mixer has specificperformance characteristics that are compatible with digitally modulatedmaterial composition. The mixer will have the ability to modulatecomposition in a volume no larger than the characteristic voxel size,perform complete mixing in the volume in roughly the time required todeposit a voxel of material at the desired material deposition rate orstaging of materials in such a way that mixing is complete beforematerial deposition. The mixer will have negligible material retentionand a near uniform residence time in the mixing chamber to eliminateaxial dispersion or cross-contamination of subsequent voxels.

A fluidic equivalent of the four roll mill can be realized usingcarefully controlled fluidic inlet/outlet elements as shown in FIG. 7.The inlets 58 and 60 receive the material and outlets 62 and 64 similarto the areas around the cylinders of the four roll mill. Fibers willgenerally orient along the principal axis, the degree of rotation in theflow can be used to control fiber rotation from pure rotation to pureextension. This is merely one example of a fluidic inlet/outletelements, other embodiments will apply within the embodiments here.

FIG. 8 shows how modulating the flow type parameter can change theprincipal straining axis. The degree of rotation of the flow ischaracterized by the parameter α that ranges from −1 indicating purerotation and +1 indicating pure extension. In FIG. 8, diagram 66 showsan orientation angle, θ, of 11.25° where α=0.040. Diagram 68 shows anorientation angle, θ, of 22.5° where α=0.172. Finally, diagram 70 showsan orientation angle, θ, of 45° where α=1.0. A fiber undergoing theflows is shown as the darker line.

An example of an inlet/outlet control element that can implement thistype of flow control. The slots at the ends of the channel serve as thefluidic inlets/outlets for the flow orientation system. Fiberstransported down the central circular channel will decelerate and orientparallel to the exit plane as they enter the inlet/outlet region. Themixed flow generated at the junction contributes to the deceleration butprimarily rotates the fiber until it orients along the principalstraining axis. This axis is controlled by the flow parameter, which ismodulated by appropriately choosing inlet and outlet flow rates in theslots.

In FIG. 9, the fluidic channel system 80 is designed to control fiberorientation. The slots 86 and 88 are the inlets for the orienting flow,and the slots 82 and 84 are the outlets as shown in the cutaway view.For fibers smaller than the deposition orifice, the fiber orientationcan be dynamically modulated at any given point in space by varying themixed flow control profile just prior to extrusion. For larger fibers,the flow in the slots is stopped and the fiber is deposited in adirection parallel to the nozzle. Orientation control of these largerfibers is possible when the print head is coupled to a three-dimensionaldeposition tool, such as the actuator 18 of FIG. 2.

The oriented particles are deposited in a three-dimensional digitalmanner in order to facilitate arbitrary particle alignment and materialcomposition. The system of FIG. 1 has a flexibility that makes itpossible to vary fiber orientation and concentration in a nearlyarbitrary manner using different techniques depending on the fiberscale. The deposition has the ability to dynamically change the angle ofdeposition relative to the substrate. The actuator 18 of FIG. 2 movesthrough all 6 axes.

Depending on the fiber length and nozzle diameters, some fibers may notbe able to flow through the nozzle without deviating from their targetorientation. In those instances, the orientation of the exit nozzle isaltered, regardless of the direction of the movement. FIGS. 10 and 11show views of the exit nozzle. FIG. 10 shows a view from above thenozzle as it deposits a fiber larger than the exit orifice. FIG. 11shows a view of a cross section perpendicular to the direction of motionof the nozzle.

In some applications, the choice of resin is an important parameter inenabling a digital workflow. In one embodiment, the resin may beUV-curable thermoset polymers that utilize acrylate linking groupsbonded to traditional thermoset monomers. These polymers possess many ofthe positive qualities of traditional thermosets but with the ease of UVprocessing. These chemistries make digital structures possible usingrasterized deposition followed by partial UV curing in a manner similarto existing inkjet-based three-dimensional printing. Alternatively, thesystem could use non-UV thermoset chemistry, such as an epoxy.

This system allows for unprecedented control of fiber orientation andconcentration. This system also allows for digital tailoring ofmechanical properties via the use of hierarchical resin systemsemploying variable amorphous and crystalline polymer domains and/ornanocomposite reinforcement used in combination with macroscopic fiberreinforcement.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A system to fabricate hierarchical gradedmaterials, comprising: a reservoir to contain a material to bedeposited; and a print head connected to the reservoir by a conduit toallow the print head to receive the material to be deposited, the printhead having a mixer to mix the material to be deposited with mixed fluidvolume control no larger than a characteristic voxel size, wherein avoxel is a three-dimensional droplet of the material, and a fluidparticle alignment section to receive the material from the mixer, thefluid particle alignment section to align fibers within the material tohave arbitrary alignment relative to at least one nozzle, wherein thenozzle is connected to the fluid particle alignment section.
 2. Thesystem of claim 1, the print head further comprising a reactive bindingsection between the mixer and the fluid particle alignment section, thereactive binding section to cause binding of particles within thematerials to link together.
 3. The system of claim 2, the reactivebinding section further comprising a stimulus, the stimulus applied tothe material in the reactive binding section to cause the particles tolink together.
 4. The system of claim 3, wherein the stimulus comprisesone of a heater and a light source.
 5. The system of claim 1, the printhead further comprising a flow focusing section to receive materialsfrom the mixer and create coaxial flow of the material.
 6. The system ofclaim 5, wherein the flow focusing section comprises a three-input flowsection.
 7. The system of claim 1, wherein the mixing section comprisesa low volume mixer with a characteristic volume on an order of a voxel,such that homogeneous mixing is performed in a volume of fluid equal toa volume of a voxel in a time equal to one of either a size of the voxeldivided by a volumetric material deposition rate or the characteristicvolume of the mixer divided by the volumetric material deposition rate,such that the residence time distribution of material in said mixer isequal to the voxel size divided by volumetric material deposition rate.8. The system of claim 1, further comprising one of a UV or thermalcuring fixture.